Pile Capacity on RQD Classification: Very Poor 1

[X-Wing]

I am designing a bridge, currently on the foundation part (Bored Pile).

The complete geotech report hasn't arrived yet, but they gave me advance copy of the borelogs that are already finished.

The classification is Rock, so no N values are given, only RQD, which ranges in Very Poor (5,15, 23% etc)

Now with only RQD as given data, how can I determine the appropriate or allowable bearing capacity on the bored piles? I have talk to someone over the phone, and he said that if the foundation is on rock, the only thing that we need to know is the "embedment length", since it is already on rocks. But I couldn't find any literature about that.

Any insights? Thanks in Advanced!!

 

[GeoPaveTraffic]

Talk to the geotechnical engineer.

Mike Lambert

 

[pelelo]

I had the same question in the past with someone.

Based on RQD values you can not design a deep foundation.

It is my understanding that at some point the designer will need to correlate or assume N values (in order to obtain friction angle). You can submit to a lab the rock cores and perform a direct shear test in order to obtain cohesion values. My opinion is RQD gives an idea of how strong or weak (or maybe how porous the rock is, if we are talking about limestone) BUT still, SPT-N values are necessary to perform more geotechcnical analysis.

That´s just my opinion.

 

[PEinc]

RQD is a measure of how fractured the rock is. It has nothing to do with strength.

http://www.PeirceEngineering.com

 

[PEinc]

Maybe this will help you:

ftp://ftp.dot.state.pa.us/public/PubsForms/Publica...

See page 573 of 1177, Section 10.7.3.2 for piles driven to rock. There is also an AASHTO reference.

http://www.PeirceEngineering.com

 

[X-Wing]

Thanks for your input sirs!

Unfortunately the geotech report wasn't still with me.

As per our structural code, if the RQD is less than 25%, it is indicated to "Use qult for an equivalent soil mass". Can anyone elaborate? Thanks!

 

[PEinc]

It sounds to me like your code is saying that if the rock is very fractured (RQD < 25%), then the rock should be treated like a very coarse, granular soil similar maybe to gravel.

http://www.PeirceEngineering.com

 

[X-Wing]

Sir if that is the case, do you have an idea on how to treat it as a cohensionless soil if only RQD is given? No N-values, undrained shear strength, etc? It if that is the case we can assume that N=100 and Cu as maximum, since it's a rock, although weak?

 

[PEinc]

RQD tells you nothing about cohesionless soil propertirs. You could estimate some conservative soil properties or find a big direct shear test box. Get unit weight, phi angle, and assume no cohesion.

http://www.PeirceEngineering.com

 

[theCorkster]

You have to make the leap from soil to rock, and you need more information besides RQD. If you don't, you'll likely be highly conservative in you design. Check out Hoek & Bray; you can use unconfined strength, RQD, and Rock Mass Rating (RMR) (based on rock properties like fracture spacing, fracture condition, fracture orientation and groundwater) to estimate insitu modulus of deformation and modulus of elasticity.

 

[PEinc]

andru18 wrote, "As per our structural code, if the RQD is less than 25%, it is indicated to "Use qult for an equivalent soil mass". The pile can be designed as a pile to rock, but that's not what his Code is requiring.

http://www.PeirceEngineering.com